Wavelength selective switch

ABSTRACT

A spectrally selective optical switch is disclosed. The switch comprises a first and a second optical waveguide each having a light guiding structure arranged to guide light along a predetermined path, the optical waveguides parallel to each other; an external resonator defined by a first and a second mirror, said first and said second mirror being provided on opposite sides and outside of said first and second light guiding structures, and said external resonator being resonant to a specific wavelength; and a deflector provided in each of said first and second optical waveguide, the deflectors being arranged to deflect light propagating in one of the light guiding structures to the other light guiding structure by operation of said external resonator. A matrix switch is also disclosed.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a wavelength selective optical switch.More particularly, the present invention relates to methods and devicesfor coupling individual channels within a wavelength divisionmultiplexed optical signal from one optical fiber to another opticalfiber.

TECHNICAL BACKGROUND AND RELATED ART

In order to increase the transmission capacity of optical fiber networksand communication links, wavelength division multiplexing (WDM)techniques are often utilized. In WDM systems, a plurality of wavelengthchannels is transmitted through a single optical fiber. Fibers areconnected at points known as “nodes”, at which channels are reroutedtowards their final destinations via the best possible fiber paths.Channels may also be added or dropped at so called add/drop points.Generally, at the nodes the WDM-signal is demultiplexed, the individualsignals re-routed and sent down the chosen fiber, possibly multiplexedinto another WDM signal. At add/drop points, spectrally selectiveoptical switches, also known as channel drop filters, are utilized forextraction of a single wavelength channel from a WDM signal, or forinsertion of a single wavelength channel into a WDM signal.

The network may be designed to be either static or re-configurable. Are-configurable network is essential to provide provision of wavelengthsand enable protection switching. In a re-configurable network, the nodesand/or add/drop points are equipped with switches or dynamic wavelengthconverters, giving it the capability to change the routing pattern.

A technology used to enable optical rerouting are MEMS switches, whichutilizes small moveable mirrors displaceable to dispatch the opticalsignal to the chosen fiber. Such an optical switch is presented in U.S.Pat. No. 6,292,281, in which a matrix of mirrors are provided on asilicon wafer based structure. Also, the nodes may be provided withadd-drop filters designed to add or drop a specific chosen channel to orfrom the WDM channel.

However, the prior art technology suffers from several drawbacks.Optical MEMS switches are complicated and difficult to manufacture, andare devices that require de-multiplexing and multiplexing of the WDMsignal.

Therefore, there is a need for devices and methods for couplingindividual channels within a WDM signal from one fiber to anotherenabling an easily re-configurable and dynamic network.

SUMMARY OF THE INVENTION

The present invention provides an optical wavelength selective switch,which eliminates, or at least alleviates, the aforementioned problems inthe prior art.

A general object of the present invention is to provide an opticaldevice for coupling one or more individual signals propagating in onefiber to another fiber. This general object is achieved by a device,arrangement and method according to the appended claims.

According to a first aspect the invention, a device is provided,comprising a first and a second optical waveguide, each having a lightguiding structure arranged to guide light along a predetermined path,the optical waveguides being arranged adjacent and parallel to eachother. Furthermore the device comprises an external resonator defined bya first and a second mirror, said first and said second mirror beingprovided on opposite sides and outside of said first and second lightguiding structures, and said external resonator being resonant to aspecific wavelength. Finally there is in each waveguide provided adeflector, which is arranged to deflect light propagating in one of thelight guiding structures to the other light guiding structure byoperation of said external resonator.

According to another aspect of the invention, the device comprises meansfor adjusting resonator wavelength and phase of the chosen channel toaffect.

In yet another aspect, the present invention can serve as an add/dropfilter. Individual WDM channels may conveniently be added or dropped bya respective channel manipulation element. Yet another field of use forthe optical device according to the present invention is in connectionwith fiber-to-fiber routers, where the present invention can providechannel exchange between two transmission fibers or fiber rings.

Moreover, the present invention provides other features and advantagesthat will become apparent when the following detailed description ofsome preferred embodiments is read and understood.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, a number of preferred embodiments of the presentinvention will be described in detail. The description below is moreeasily understood when read in conjunction with the drawings, in which

FIG. 1 schematically shows a first embodiment of a basic optical switchelement according to the present invention;

FIG. 2 schematically shows an embodiment of a cleaning device that mayadvantageously be used together with the switch element;

FIG. 3 schematically shows an embodiment of a add/drop device using anarray of switch elements;

FIG. 4 schematically shows another embodiment of a add/drop device usingan array of switch elements;

FIG. 5 schematically shows yet another embodiment of a add/drop deviceusing an array of switch elements;

FIG. 6 schematically shows yet another embodiment of a add/drop deviceusing an array of switch elements;

FIG. 7 schematically shows an switch device incorporating a switchcontroller; and

FIGS. 8-12 3 schematically shows embodiments of various switcharrangements.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A first preferred embodiment of an optical switch element 1 according tothe present invention is schematically shown in FIG. 1. The arrangementshown is to be regarded as the best mode of carrying out the invention.The switch element is used to switch an optical signal from one opticalfiber to another optical fiber. This element fundamentally issimultaneously wavelength selective, wavelength tunable and arraycascadable.

The switch element 1 comprises a first optical fiber or waveguide 2 anda second optical fiber or waveguide 3, each having a fiber core denoted4 and 5, respectively. The function of the optical fibers could also beimplemented using other sorts of waveguides in glass material orsemiconductor material. Considering optical fibers, there could be usedtwo separate fibers as depicted in FIG. 1 or a single fiber with a dualcore could be used. The cores 4, 5 are provided with a first deflector 6and a second deflector 7, respectively. Each deflector 6 and 7 comprisestwo superimposed blazed fiber Bragg gratings oriented at right angleswith respect to each other. The deflectors 6, 7 deflect light impingingupon the two superimposed fiber Bragg gratings into two anti-parallelbeams. The deflector element could also be implemented using other sortsof deflectors, e.g. simple blazed fiber Bragg gratings, angledconcentrated mirrors, such as metallic mirrors dielectric step mirrors,or angled Bragg reflectors, such as angled dielectric stack mirrors orblazed-grated waveguides. The switch element shown in the figure furthercomprises two external mirrors 8 and 9, forming an external Fabry-Perottype resonator 10. The external resonator is positioned so that thedeflectors 6, 7 are enclosed within the resonator. The externalFabry-Perot resonator may also be implemented using concentratedmirrors, such as metallic mirrors or dielectric step mirrors, or Braggreflectors, such as dielectric stack mirrors or grated wave-guides.Finally the switch element includes actuators 11, 12 operative to changethe optical length of the external resonator (the optical distancebetween the mirrors) to provide wavelength and phase tuning of theswitch element. The optical distance comprises the distance between thefirst external mirror and the first deflector, distance between thefirst deflector and the second deflector and the distance between thesecond deflector and the second external mirror. Each of the distancesmay be arranged to be individually tunable. The actuators 11, 12 couldbe implemented using various sorts of actuation methods for changing theoptical path length: Actuation by changing the geometrical path length,such as piezo or electrostatic actuation, or actuation by changing therefractive index, such as current injection or reverse-bias actuation inthe p-n junction of a semiconductor.

The external resonator provides a spectrally selective enhancement oflight energy in the region intended for switching. This is in order toselect the particular channel of interest for switching. This is whileleaving other channels substantially unaffected. The external resonatorforms a Fabry-Perot cavity in which the selected light wavelengthexperience spectral resonance. The spectral resonance is due toconstructive interference by multiple reflections between the first anda second external mirror. The optical path distance between the externalmirrors is in the following called the cavity optical length. The cavityoptical length multiplied by a factor of two equals a multiple integernumber of wavelengths, for the selected channel. Thus, the amount ofswitched light energy for the channel external resonator depends on thematching of the wavelength to the Fabry-Perot resonance and on thevalues of reflectance of the external mirrors. Thus, the external mirrorprovides spectral selection. This is in the following denoted thewavelength tuning mechanism. The wavelength tuning mechanism is a subproduct of the Fabry-Perot mechanism.

The deflectors 6, 7 provides the coupling of light energy from the firstoptical fiber 2 to the second optical fiber 3, which is enhanced for thechosen channel by the wavelength tuning mechanism of the externalresonator. Here, the switching is obtained as follows. First, light inthe first optical fiber 2 is deflected by the deflector 6 into theexternal resonator 10. Second, resonance of the selected wavelength isachieved due to the Fabry-Perot resonator. Third, a second deflection ofthe chosen channel from said external resonator by the second deflector7 into the second optical fiber.

Another underlying mechanism, in the following denoted the phase tuningmechanism, is provided by the innovative use of superimposed blazedgratings. This provides a means to obtain the phase tuning mechanism inaddition to the wavelength tuning mechanism.

The phase tuning mechanism provides a means of turning the switchelement from a “bar” to “cross” state. This without the need employ thewavelength tuning mechanism to detune (change) the resonant wavelength.Thus, in a cross-state of the switch element the selected wavelength iscoupled from the first to the second optical fiber. In the bar state,the selected wavelength is not coupled to the second fiber, butcontinues to propagate in the first fiber. The switch element can alsobe tuned, by the phase tuning mechanism, to a ‘broadcast’ state, beingan intermediate state between the bar and cross states. In the broadcaststate, the resonant wavelength from the input of the fiber is droppedjust in fraction to the second fiber, while the remainder of the lightcontinues to propagate in the first fiber. Thus, the amount of switchedlight energy does depend on the phase tuning mechanism. This since theamount of switched light energy does depend also on the phase relationsin interference, which in turn depends of the following optical pathdistances: the distance between the first external mirror and the firstdeflector, the distance between the first deflector and the seconddeflector, and the distance between the second deflector and the secondexternal mirror.

The following set of physical properties of the switch element should beconsidered as a set of critical design parameters of the device. Inorder to achieve a certain free spectral range of the device, i.e.leaving the spectral properties of the light unchanged except for thespectral region of interest in which the switching device is to operate,a certain maximum geometrical distance between the mirrors must be set.The radii of curvature of the mirrors can be used for compensation of anasymmetry of the optical field distribution inside the cavity, and canalso be used for enhancing the coupling efficiency and overallperformance of the switching device, e.g. by compensating for certaingeometrical limitations of the cavity. The angle of out- and in-couplingof the optical modes in the fibers is to be chosen as close as possibleto the perpendicular direction of the symmetry axis of the fiber, stillleaving the cone of out- or in-coupled light free from the perpendiculardirection. In case the perpendicular direction is contained within thecone of out- or in-coupled light, a spectral degeneracy of the couplingefficiency of the switch will occur, causing an unwanted enhancement ofcoupling efficiency in a region of the spectrum of operation of theswitch. The tilt of the mirrors relative to each other (deviation froman all-parallel configuration of the cavity mirrors) control thespectral width of the coupling between the optical modes in the fibers.The more parallel the mirrors are to each other, the narrower andenhanced the spectral coupling efficiency will be, and, vice versa, themore tilt relative each other the mirrors possess, the wider and weakerthe spectral coupling efficiency will be. The length of the blazedgratings of the optical fibers determines the effective length ofinteraction between the modes propagating in the optical fibers. Thedesired length of the blazed gratings is determined from the couplingstrength of the gratings and the desired angular width of the out- orin-coupled light cone. For gratings possessing high couplingefficiencies, a short grating length should be chosen in order not tocause unwanted coupling back into the original fiber at the end of thegrating. Similarly, for weak gratings, the grating length should bechosen long enough in order to ensure that the light coupled out fromthe first fiber is fully coupled into the second fiber. In addition, thelength of the gratings determines the angular width of the cone of out-or in-coupled light of the fibers. The longer the grating length, themore narrow the cone of out- or in-coupled light, and vice versa. Theangular width of the out- or in-coupled light of the fibers, asdetermined by the grating length as previously described, determine howclose to the perpendicular direction of the fiber one may choose out-and in-coupling light cone. The more narrow the cone is, the more closeto perpendicular one may choose the direction of out-coupling, henceincreasing the coupling efficiency between the interacting optical modesin the fibers. In order to achieve a high coupling efficiency betweenthe propagating optical modes in the fibers, the reflectivities of themirrors of the cavity should be chosen as high as possible.Imperfections of the mirror surfaces will cause scattering of thereflected optical waves inside the cavity, hence effectively causing aloss of power and a decrease in coupling efficiency. The strengths ofthe gratings are related to the length of the gratings.

In FIG. 2 a cleaning element 20 is depicted. Such an element may bedesirable to incorporate into switch devices to ‘clean’ a lightwavelength channel from remaining signal content, down to a very lowsignal power level. Channel cleaning may be required in order tocompensate for a non-ideal switching element. This since the switchingelement may not be able to completely switch all signal content in alight wavelength, when aiming to drop this light wavelength from thefirst to the second waveguide. The first waveguide should be cleanedfrom the unintentionally remaining signal content. This in order toallow re-use of the same wavelength for another data signal, whileavoiding coherent mixing of the old and the new data signals sharing thesame wavelength position. The cleaning element may be implemented in thesame manner as the switch elements. Here, however, a second waveguide isnot required to pick up out coupled light.

Yet another arrangement may be used to obtain a spectrally cleanswitching element. In this arrangement, two switch elements are used ina serial configuration as subsequently described. In the spectrallyclean switching configuration, the first waveguide of the first basicswitch element is connected to the first waveguide of the second basicswitch element via a waveguide in which one may control the optical pathlength over which the light propagates from the first to the secondbasic switch element. The second waveguide of the first switchingelement is similarly connected to the second waveguide of the secondswitch element via a waveguide whose optical path length may, but notnecessarily have to, be possible to control. The interconnectingwaveguides, at least one of which possess the possibility of controllingthe optical path length experienced by a propagating light wave, aretogether denoted as the enabling element of the spectrally cleanswitching element. As subsequently described, the role of the enablingelement is to turn on or off the switching of a particular spectralregion of the light, as determined by the configuration of the two basicswitching elements. The two basic switch elements should, in the case ofconstituting the two switching components of a spectrally cleanswitching element, be designed in such a way that only fifty percent ofthe light in the first waveguide of each basic switch element is coupledover to the second waveguide of respective basic switch element.

The principle of operation of the spectrally clean switching element isas follows. As fifty percent of the power of the light in the firstwaveguide of the first basic switch element is coupled over to thesecond waveguide, passing through the waveguides of the enablingelement, and being recombined with the remaining fifty percent of thelight power in the second basic switch element, the relative phase ofthe light waves, when recombined in the second switching element, willdetermine whether constructive or destructive interference occur in theblazed gratings of the second basic switch element. When the relativephase shift of the light waves in the waveguides, when entering thesecond basic switch element from the enabling element, is a multiple of2*pi, the second basic switching element will cause the remaining fiftypercent of the light power in the first waveguide to be coupled over tothe second waveguide, hence causing a switching from the first to thesecond channel of the spectrally clean switching element. In this case,the enabling element acts as an optical interconnect leaving therelative phase of the light waves invariant, and the spectrallyselective switching is performed in the same manner as in the case of abasic switching element nominally designed for switching one hundredpercent of the optical power in a certain spectral region. On the otherhand, when the relative phase shift between the channels is pi plus amultiple of 2*pi, the second basic switching element will cause thefifty percent light power in the second waveguide to be coupled backinto the first waveguide, hence in the first case, for a N*2*pi relativephase shift, a constructive interference is experienced in the secondwaveguide of the second basic switching element, while in the secondcase, for a (2*N+1)*pi relative phase shift, a constructive interferenceis instead experienced in the first waveguide. Using the enablingelement as an on/off control (enabler) of the switch, the describedprinciple of operation allows the basic switch elements to bereconfigured or adjusted in the off state, with a N*2*pi relative phaseshift between the waveguides in the enabling element, without affectingthe light waves in any of the neighboring wavelength channels. As thereconfiguration or adjustment of the basic switch elements is done, thespectrally clean switching element is put into operation by tuning therelative phase shift between the waveguides to (2*N+1)*pi. As the phaseshift is tuned, the optical power coupled from the first to the secondwaveguide of the spectrally clean switching element will graduallyincrease from nominally zero to one hundred percent. The spectral regionin which the switching is performed is confined to the interval asdetermined by the configurations of the two basic switching elements,and since these are kept constant during the tuning of the enabler,adjacent wavelength channels are left unaffected by the spectrally cleanswitching element.

Several mechanisms may be considered for achieving a relative phaseshift between the light in the two waveguides of the enabling element.One way of causing the optical path length in one waveguide to change isto apply mechanical stress to the waveguide. Other means of changing theoptical path length is to employ electro-optical effects,magneto-optical effects, or by other means, chemically, mechanically, orelectro-magnetically, changing the refractive index of the core and/orcladding of the waveguides of the enabling element. The enabling elementmay also be incorporated in the first or second basic switch element byapplying an electro-optical layer to any of the mirrors of the cavity,and applying a weakly reflecting mirror to the electro-optical layer, ina setup as for a dynamically reconfigurable Gires-Tournoisinterferometer.

In may be desired to incorporate polarization management, since theswitch element and the cleaning element inherently is unable to properlyhandle light with arbitrary polarization state. Proposed devicesincludes Faraday mirror polarization handler, quarter-wave plate mirrorpolarization handler, dual path polarization handler and serial twistpolarization handler.

FIG. 3 shows an add-drop device where polarization management isobtained by having positioned between a circulator 30 and a pair ofFaraday mirrors 31, an array of switching elements 32. Input light thuspasses the circulator of the first waveguide. In the switch element, forthe selected wavelength, i.e. the light wavelength which is resonant tothe external resonator, when in cross state (i.e. when the phase tuningmechanism is used to achieve the cross state) one polarization componentof this light wavelength is switched over to the second waveguide. Sincethe switch element is polarization selective, the remaining,perpendicular, polarization component is unaffected by this switchelement, and passes through to the Faraday mirror of the firstwaveguide. Here, the light wavelength is back reflected. However, thepolarization component has been rotated by 90 degrees by the Faradaymirror. Thus, when again reaching the same switch element, theback-reflected light now has the proper polarization state to beswitched to the second waveguide. Due to the properties of the switchingelement, after having been coupled to the second waveguide, thedifferent switched portions of the light wavelength will have the samepolarization state but travel in opposite directions. Also, coherentmixing can be avoided within the switch element by proper design. Whenleaving the switch element, the polarization component, which wasdirectly coupled from the first waveguide to the second waveguide, willbe back reflected and rotated 90 degrees in the Faraday mirror of thesecond waveguide. This polarization component then passes the switchelement without experience switching, since it has now been rotated tothe insensitive polarization state, and following propagates to thecirculator of the second waveguide. However, the polarization component,which was coupled from the first waveguide after Faraday mirror backreflection and rotation to the second waveguide, will be propagatingdirectly to the circulator of the second waveguide. The two polarizationcomponents thus again combine in the second waveguide, as two orthogonalstates of polarization. It is required that the optical paths betweenthe switch element and the Faraday rotators of first and secondwaveguides are matched. This is in order to avoid polarization modedispersion and also to avoid polarization dependent loss.

In the case of the Faraday mirror polarization handler, it is notrequired for polarization states to be maintained when light ispropagating in waveguides between the switching element and therespective Faraday mirrors. This is due to reciprocity. It is sufficientthat the polarization properties of the fiber, does not alter within thetime it takes for light to propagate from the switching element to therespective Faraday mirror and back again. This condition is typicallyfulfilled, since the light propagation time is typically on the order ofpicoseconds, while the polarization changes are typically very muchslower.

In FIG. 4 the Faraday mirror is replaced with a common quarter waveplate mirror 41. For a quarter-wave plate mirror polarization handler,the polarization state must be substantially maintained between theswitching element(s) and the quarter-wave plate with mirror. Also, thepolarization state for a switched wavelength must be substantiallylinear after having dropped one polarization component. Further, theremaining polarization component must be aligned at 45 degrees withrespect to the optical axis of the quarter-wave plate. This in order forthe quarter-wave plate and the mirror to return the remainingpolarization rotated 90 degrees and thus switched by the element at thereturn path. The advantage with the quarter-wave plate mirrorpolarization handler is that a common quarter-wave plate and mirrorcould be more easily employed for both the first and second waveguide.This is not so easily implemented when using Faraday rotator elements inthe Faraday mirror handler.

In FIG. 5 an add-drop device using dual path polarization handler isdepicted. This is obtained by having the polarization of the input lightpropagating in the first waveguide split by a polarizing beam splitter51 into two components, before the switching elements. Thus eachpolarization state is injected into a respective first waveguide, whichis substantially polarization maintaining. Following, each polarizationcomponent is handled in its respective first waveguide. Each firstwaveguide contains one or more switching elements. After the switchingelements, the respective first waveguides are again combined usinganother polarizing beam splitter 52 to a common first waveguide. Thus,for light wavelengths, which are not switched, the polarizationcomponents are again combined after having propagated through theswitching elements. The arrangement of the second waveguide is a mirrorimage of the first waveguide. Thus, for light wavelengths which areswitched, the polarization components are again combined after havingpropagated through the switching elements. The advantage of the dualpath polarization handler is that it works in transmission, and thusdoes not require circulator components. A disadvantage with the dualpath polarization handler is that it requires at least two switchingelements for each wavelength, since each of the two polarizationcomponents requires a dedicated switching element. Also, thepolarization has to be substantially maintained, considering allwaveguides. Further, if employing a switch controller, both polarizationcomponents might need to be controlled independently.

In FIG. 6, an add-drop device using serial twist polarization handlingis shown. This is obtained by having the polarization of the input lightpropagating in the first waveguide, propagating to a first series ofswitch elements for different light wavelengths. The first array ofswitch elements then acts upon one of the polarization components. Thisin order to either switch or not switch a particular light wavelength.Then, the remaining polarization component, to be acted upon, is rotated(twisted) substantially 90 degrees, relative to a second array ofswitching elements. Following, the remaining polarization component isacted upon by a second array of switch elements. For the serial twistpolarization handler, the rotation (twisting) of the remainingpolarization component could be accomplished by twisting the mainpolarization axis of both the first and second waveguides. Here, thefirst and second waveguides are required to substantially maintain thepolarization state of the propagating light wavelengths. The advantageof the serial twist polarization handler is that it works intransmission, and thus does not require circulator components. Adisadvantage with the serial twist polarization handler is that itrequires at least two switching elements for each wavelength, since eachof the two polarization components requires a dedicated switchingelement. Also, the polarization has to be substantially maintained,considering all waveguides. Further, if employing a switch controller,both polarization components might need to be controlled independently.

FIG. 7 shows an integrated switch device 70, incorporating the inventiveoptical switch. The switch device 70 includes a switch controller 71that monitors the light output of the switch elements and uses thisinformation as a feedback for adjustment. Light is coupled from theinput optical fibers via tap devices 72. Which input fiber to bemeasured is then chosen by the mechanical switch 73. In general, aswitch controller measures the output light wavelengths and possiblyalso the input light wavelengths for the first and second waveguides ofthe switch element(s). The switch controller uses this information tocontrol the states of the actuation elements, such that the desiredsignal power is obtained for the respective output light wavelengths.

In order to fully handle a given switch application, the correspondingimplementation of a full-featured integrated switch device, isdesirable. Such an integrated switch device may consist of an assemblyof switch elements, polarization handlers, cleaning elements and switchcontrollers.

In FIGS. 8-12 a number of types of basic device implementations for theswitch mechanism are shown. Such a device is referred to as anintegrated switch device. An integrated switch device may consist of anassembly of underlying elements, such as switch elements, polarizationhandlers, cleaning elements and also switch controllers.

In FIG. 8 a wavelength-selective 2×2 fiber switch 80 is depicted. It isbuilt by underlying devices, of the types add-drop device 81 andcleaning device 82. This integrated switch device is denoted thetwo-fiber switch device.

Note that when the two-fiber switch device employs the dual pathpolarization handler, the number of polarizing beam splitters can bereduced. This is by splitting the polarization components at the inputports of the two-fiber switch device, and keeping them separated untilthe output ports, where polarizing beam splitters are used to againcombine the polarization components. Here, in order to avoidpolarization mode dispersion, the optical paths have to be carefullymatched for the respective polarization components.

Note that when the two-fiber switch device employs the serial twistpolarization handler, the number of polarization twisters can bereduced. This is by first handling dropping, cleaning and adding of oneof the polarization components, then employ a polarization twister, andthen second dropping, cleaning and adding the remaining polarizationcomponent. Here, in order to avoid polarization mode dispersion, theoptical paths have to be carefully matched for the respectivepolarization components.

In FIG. 9 a multiplex device 90 is depicted. This devicemultiplexes/de-multiplexes one ingress fiber from/to several egressfibers.

In FIG. 10 a matrix device 100 is depicted. The device type uses N inputfibers to N output fibers, where the input fibers are crossed withrespect to the output fibers. The N input fibers are linked to the Noutput fibers in N*N nodes. The linking occurs via switch elements. Thisswitch device is referred to as a matrix switch device. For the matrixswitch device, a configuration could be chosen where light wavelengthsare exchanged between the fibers via two-fiber switch devices. Note thatwhen the matrix switch device employs the dual path polarizationhandler, the number of polarizing beam splitters can be reduced. This isby splitting the polarization components at the input ports of thematrix switch device, and keeping them separated until the output ports,where polarizing beam splitters are used to again combine thepolarization components. Here, in order to avoid polarization modedispersion, the optical paths have to be carefully matched for therespective polarization components. Note that when the matrix switchdevice employs the serial twist polarization handler, the number ofpolarization twisters can be reduced. This is by first handlingdropping, cleaning and adding of one of the polarization components,then employ a polarization twister, and then second dropping, cleaningand adding the remaining polarization component. Here, in order to avoidpolarization mode dispersion, the optical paths have to be carefullymatched for the respective polarization components. For the matrixswitch device, alternatively, another configuration could be chosenwhere the input and output fibers are perpendicularly oriented relativeto each other in the basic switch elements. Half-wave plates could thenbe inserted in the switching elements, between the input fibers andoutput fibers. This since the switched polarization component needs tobe rotated 90 degrees to be oriented perpendicular to the propagationdirection of the output fiber.

FIG. 11 depicts a switch device, which uses multiple cascaded stages oftwo-fiber switch devices. This sixth switch device will be referred toas a the multi-stage switch device.

FIG. 12 depicts a close network type switch device using two-fiberswitch devices.

The embodiments of the present invention that are described above andschematically shown in the drawings are not intended to limit the scopeof the protection sought. On the contrary, any person skilled in the artwill realize that a number of different embodiments, and modification ofthe embodiments shown and described, are conceivable within the scope ofthe invention. The scope of the invention is defined in the appendedclaims.

1. A spectrally selective optical switch, comprising a first and asecond optical waveguide each having a light guiding structure arrangedto guide light along a predetermined path, the optical waveguides beingarranged adjacent and parallel to each other; an external resonatordefined by a first and a second mirror, said first and said secondmirror being provided on opposite sides and outside of said first andsecond light guiding structures, and said external resonator beingresonant to a specific wavelength; and a deflector provided in each ofsaid first and second optical waveguide, the deflectors being arrangedto deflect light propagating in one of the light guiding structures tothe other light guiding structure by operation of said externalresonator.
 2. The optical switch according to claim 1, wherein thedeflector in at least one of the waveguides comprises a first tiltedreflector arranged in said waveguide, and a second tilted reflectorarranged in said waveguide, wherein said first and said second tiltedreflectors are superimposed upon each other, and arranged to deflectlight out from said waveguide into two individual beams.
 3. The opticalswitch according to claim 1, wherein each tilted reflector comprises ablazed Bragg grating.
 4. The optical switch according to claim 1,wherein either one of the first and the second mirror is a dielectricmulti-layer mirror.
 5. The optical switch according to claim 1, whereinthe wavelength to which the external resonator is resonant isadjustable, the spectrally selective optical switch thereby beingtunable.
 6. The optical switch according to claim 1, wherein the opticalwaveguide is an optical fiber and the light guiding structure is a corein said optical fiber.
 7. The optical switch according to claim 1,wherein the first and second waveguides are implemented in the form of adual-core fiber.
 8. A matrix switch device, which uses N input fibers toN output fibers, where the input fibers are crossed with respect to theoutput fibers and where the N input fibers are linked to the N outputfibers in N*N nodes, wherein said linking is at least partlyaccomplished with an optical switch according to claim
 1. 9. Anarrangement comprising two optical switches as defined in claim 1,wherein the first optical waveguides of the switches are connected toeach other by means of a first interconnecting waveguide and the secondoptical waveguides of the switched are connected to each other by meansof a second interconnecting waveguide, and wherein each of said switchesis arranged to couple 50 percent of available light power from the firstoptical waveguide to the second optical waveguide, the arrangementfurther comprising means for altering the optical path length of atleast one of the first and the second interconnecting waveguides suchthat constructive or destructive interference can be obtained in thesecond optical waveguide of the second switch by altering said opticalpath length.